1. Field of the Invention
The present invention relates to a method and apparatus for sensing pressure and, more particularly, to such a method and apparatus which are operable to permit the sensing of the internal pressure of pressurized work objects, such as the sensing of intraocular pressure, in a manner which is substantially more precise than has heretofore been possible.
2. Description of the Prior Art
The eye, or more precisely the eyeball, of humans and other living creatures, is a pressurized spherical vessel. The pressure, known as "intraocular pressure", serves to maintain the shape of the eyeball against collapse from ambient atmospheric pressure as well as acting in a variety of different ways to maintain the normal physiological functioning of the visual system. For example, intraocular pressure maintains the refracting capability of the cornea, supplies nourishment to the corneal layers, assists in retaining the stability of the retina, sclera and intraocular blood vessels, among its many functions. Thus, the maintenance of intraocular pressure within a normal range is of substantial importance for a number of significant reasons.
Abnormal variations in intraocular pressure are symptomatic of a host of diseases and abnormal physical conditions requiring treatment. Thus, the sensing of abnormal variations in intraocular pressure is critical to the diagnosis of such conditions. For example, glaucoma is a disease of the eye in which the aqueous humor of the eye does not drain normally resulting in an increase in intraocular pressure. If untreated, the condition ultimately destroys the optic nerve causing blindness.
The normal range of intraocular pressure is from about 10 to about 21 millimeters Hg. with the means being 16 millimeters Hg., plus or minus 2.5 millimeters Hg. However, intraocular pressure is subject to circadian rhythm such that, for example, the highest intraocular pressure for a given person is at about 6:00 a.m. In addition, the intraocular pressure is varied by blood pressure, heartbeat, respiration, season of the year, caffeine, other chemical substances, thyroid eye disease and a host of other factors. As a consequence, intraocular pressure is constantly fluctuating in reaction to all of these factors. When it is necessary or advisable to determine the intraocular pressure, it is impossible to predict when the most representative reading, for the particular purpose for which the intraocular pressure is to be checked, should be taken. Any single reading may thus be misleading and multiple readings taken at the same time every day, or on different equipment, or by different observers, or under different physiological or pathological conditions, may similarly be misleading. Thus, it has long been known that it would be highly advantageous to obtain both multiple and continuous readings over a lengthy period of time and extending over several days to as long as several weeks.
There are several conventional methods for measuring intraocular pressure. These can generally be categorized as direct and indirect methods. For example, in applanation tonometry, the force required to flatten 3.06 millimeters of the cornea is measured to provide the reading. Other electronic devices use smaller areas of contact. However, all such prior art methods are limited by their inability to provide a continuous reading over any substantial period of time; that is, they require the patient's presence at the test facility where the test equipment is located. Therefore, typically several individual measurements are taken over several days. This is both inconvenient to the patient and misleading for all of the reasons previously noted. There has not previously been a method by which, as a practical matter, the intraocular pressure could continuously be monitored over a length of time sufficient to provide reliable readings from which to diagnose disease or upon which medical decisions could be made. This absence of reliable data is particularly critical in the monitoring of low tension glaucoma and is even more acute where the patient is taking medication and where the other influencing conditions noted above prevail.
Therefore, it has long been known that it would be desirable to have a method and apparatus providing a means by which the internal pressure of work objects, such as the intraocular pressure of the human eye, could be determined over prolonged test periods to provide reliable data useful in detecting diseases such as glaucoma and abnormal physical conditions associated with the deviation from normal of intraocular pressure in a manner which is substantially more accurate and dependable than heretofore possible while being of little or no discomfort to the patient.